The present invention relates generally to integrated circuit devices, and more specifically, to Low Temperature Co-fire Ceramic Systems.
Low Temperature Co-fire Ceramic Systems (LTCC) are a low cost, high performance solution for ceramic packaging of integrated circuit devices. LTCC integrated circuit devices are generally formed of multiple ceramic material layers, within which are embedded electrically conductive pathways and electronic circuit components, such as resistors and capacitors. Typically, ceramics with low dielectric constants are desirable for these applications and highly conductive metals, such as Ag, Cu, and Au, are used as metallization materials. LTCC systems offer excellent dielectric isolation, high layer count circuitry, high performance conductors, and inexpensive processing. These characteristics lead LTCC semiconductor devices to have high densities, reliability, and performance, at low costs. In some applications, semiconductor devices may be electrically mounted onto LTCC integrated circuit modules.
In order to illustrate the structure of an LTCC module in greater detail, FIG. 1 illustrates a side plan, cross-sectional view of a portion 100 of an LTCC module. This module may be for example, a ball grid arrayed type module. This specific LTCC module is formed of four ceramic material layers 102, 104, 106 and 108. Layer 102 is referred to as the bottom layer because it will be adjacent to a printed circuit board (PCB) upon mounting of the LTCC module. Respectively, layer 108 is referred to as the top layer. The electronic components (not shown) within the LTCC module are formed on the surfaces of each ceramic layer. The electronic components on the various layers are then electrically connected to each other through the conductive traces 110 that run along the surface of each layer, and the conductive material that fills the holes or through-holes that pass through the thickness of each layer. The conductive material filling the holes form vias 112. Catch pads 114 are formed at the bottom end of each of the vias 112 to facilitate a solid connection between vias 112 and conductive traces 110 on the lower and adjacent ceramic layer or with contact (or solder) pads 115. Catch pads 114 also make it easier to fill vias 112 with conductive material. It is noted that catch pads 114 facilitate solid connections but are not necessary to form connections between conductive traces 110 and vias 112. Contact (or solder pads) 115 are formed on the bottom surface of layer 102 and are connected to catch pads 114. Contact pads 115 provide a surface onto which solder balls 116 can be formed. Solder balls 116 are used to connect LTCC module 100 to an electronic substrate, such as a printed circuit board.
Even though LTCC modules currently provide a low cost, high performance solution for ceramic packaging of integrated circuit devices, continuing efforts are underway to improve this technology. One area of specific importance relates to the durability of the LTCC modules upon attachment to printed circuit boards. Unfortunately, a problem arises in this circumstance since the coefficient of thermal expansion for LTCC modules is much lower than that of standard printed circuit boards. This results in thermal cycling fatigue due to the relatively large degree of expansion and contraction of the printed circuit board, as compared to that of the LTCC modules. Ultimately, fatigue failure manifests itself when the ceramic material surrounding the contact pads and/or the contact pad cracks and thereby causes contact pads 115 to fall out from the LTCC module. It is noted here that solder balls 116 usually become melded into a contiguous formation with contact pads 115 during the LTCC manufacturing process, and therefore, solder balls 116 separate from the LTCC module together with the contact pads 115. Simply put, such structural failure ends the useful life of an LTCC module.
In view of the foregoing, a solution for increasing the useful life of LTCC integrated circuit modules by strengthening the bond between the contact pads and the ceramic material of the LTCC module would be desirable.
The present invention is directed to a structural design and a method for forming LTCC integrated circuit modules that exhibit long thermal cycle fatigue lives after being attached to electronic substrates, such as printed circuit boards. The present invention thereby greatly increases the utility and value of LTCC modules. An LTCC module according to one embodiment of the present invention comprises a plurality of ceramic layers and at least one electronic component embedded within the plurality of ceramic layers. Within a first one of the ceramic layers is a hole that passes through the ceramic layer, the hole being filled with a first electrically conductive material to form a via. A contact pad is formed on a surface of the ceramic layer, wherein the contact pad formed from a second electrically conductive material that is different from the first electrically conductive material. A barrier cap is formed between the via and the contact pad, wherein the barrier cap is formed from a third electrically conductive material that is different from the first and second electrically conductive materials. A dielectric ring covers a peripheral portion of the contact pad and an adjacent portion of the dielectric material layer surface immediately surrounding the contact pad, such that any solder that is applied to the contact does not contact the peripheral portion of the contact pad or the ceramic material.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.